1. Field of the Invention
The present invention generally relates to a surface acoustic wave transducer, and more particularly to a surface acoustic wave unidirectional transducer having interdigital electrodes and floating electrode arranged between successive interdigital electrodes.
There has been known a surface acoustic wave transducer comprising a piezoelectric substrate and interdigital electrodes applied on a surface of the piezoelectric substrate. Such a transducer having the interdigital electrodes is called IDT and has been widely used as various kinds of filters for a mobile communication system.
2. Related Art Statement
There have been proposed various IDT electrode arrangements. For instance, Japanese Patent Publication Tokko Hei 3-20929, there is described a known IDT electrode arrangement, in which open-type or short-circuit-type floating electrodes are provided between successive positive and negative electrodes such that the floating electrodes are not brought into contact with the positive and negative electrodes. In such a known IDT electrode arrangement, a direction of a surface acoustic wave reflected by the open-type or short-circuit-type floating electrodes becomes unidirectional.
There have been proposed various kinds of the unidirectional transducers such as a unidirectional transducer using a multistrip coupler, a multi-phase type unidirectional transducer and a single-phase type unidirectional transducer (refer to Japanese book, "Fundaments and Applications of Microwave Circuit", published on Aug. 20, 1990 from Sougou Densi Shuppan, page 280). The known IDT electrode arrangement disclosed in the above mentioned Japanese Patent Publication belongs to the single-phase type unidirectional transducer utilizing the reflection by both the open-type and short-circuit-type floating electrodes.
The reason why the unidirectional transducer using the floating electrodes has a large unidirectional property is as follows. A reflecting coefficient of a metal strip electrode applied on a piezoelectric substrate varies depending upon the thickness, width, etc. of the metal electrode and a phase of a surface acoustic wave reflected by a short-circuit-type floating electrode differs from a phase of the wave reflected by the open-type floating electrode by 180 degrees. When the open-type floating electrode is formed at a position which is deviated from a middle point between adjacent positive and negative exciting electrodes of IDT, there is produced a shift between, an exciting center of the surface acoustic wave and a reflection center, so that the directionality can be attained. When the short-circuit-type floating electrodes are used, the reflection phase is reversed, so that the reversed directionality can be obtained. Therefore, when two floating electrodes are provided, there are obtained large reflection and directionality.
The above explained surface acoustic wave transversal filter has various properties. That is to say, since the propagating velocity of the surface acoustic wave is lower than that of the electromagnetic wave, the device can be small in size and light in weight. Further, the signal tapping on the substrate can be performed easily and the shape of the IDT electrode arrangement corresponds directly to the impulse response.
There have been various known methods of weighting the electrodes corresponding to the impulse response. For instance, a length of portions of adjacent positive and negative electrodes and floating electrodes which are opposed to each other viewed in a direction of the electrode array, i.e., a wave propagating direction is changed such that the length is maximum at a middle of the electrode array and is gradually decreased toward ends of the electrode array so that the exciting intensity is suitably distributed along the direction in which the electrodes extend. Hereinafter, said length of portions of electrodes which are opposed to each other is sometimes called an opposing length of electrodes. This weighting method is usually termed as the apodize method. In an axial direction weighting method, the opposing length of electrode is made constant over the whole electrode array and an intensity of an exciting electric field is changed in the direction of the electrode array such that the electric field becomes maximum at a middle of the electrode array and is gradually decreased toward both ends of the electrode array. A similar function can be attained by deleting some electrodes from the electrode array. This later weighting method is called the electrode withdrawal weighting method.
FIG. 1 is a plan view showing a known surface acoustic wave transducer. This transducer 1 comprises a substrate made of a piezoelectric material, an input or transmission side IDT electrode structure 2 and an output or reception side IDT electrode structure 3, the input and output side IDT electrode structures 2 and 3 being formed on a surface of the substrate 1. Electrodes of the input side IDT electrode structure 2 are weighted in accordance with the apodize method and electrodes of the output side electrode structure 3 are of normal type.
In the known unidirectional surface acoustic wave transducers including the transducer shown in FIG. 1, a component having a desired frequency could not be derived efficiently and an insertion loss could not be reduced sufficiently.